Leave a dotted key out of the runs export header

_leaves named a column for a key like `-1.0` that _dig can never read
back, so the export showed an always-empty column. Skip such a key.

backend/tests/api/routes/test_runs.py

@@ -1121,6 +1121,9 @@ def exported():
                         "note": "n/a",
                         "final_metrics": {"train_loss": acc * 2},
                         "test_metrics": {"known": {"perfect": 1.0}},
+                        # Keyed by a number written with its decimal point,
+                        # which no dotted path can address.
+                        "spectrum": {"-1.0": acc, "0.25": {"mean": acc}},
                     },
                     created_at=made + timedelta(seconds=index),
                 )
@@ -1199,6 +1202,8 @@ def test_an_exported_run_row_carries_every_recorded_input(
     assert rows[0]["metric.final_metrics.train_loss"] == 0.5
     assert rows[0]["metric.test_metrics.known.perfect"] == 1.0
     assert "metric.note" not in rows[0]
+    # A key holding the separator cannot be read back, so it is not a column.
+    assert not any(key.startswith("metric.spectrum") for key in rows[0])
 
     named = _lines(export(format="jsonl", metrics="test_metrics.known.perfect"))
     assert list(named[0])[-1] == "metric.test_metrics.known.perfect"